I'm fairly confident that you are:
Countless men spilled their blood over me
Countless men spilled their blood over time. In fact, every single man who had their blood spilled.
Some called me a gift, some called me a burden
The gift of time, or the burden of time, refers to how one chooses to interpret the passage of time in their lives.
Tears were shed at the simple sight of my form
Little dubious, but could refer to people crying over the time that has passed, or when they miss some date to something important, or over the joy of memories of past times.
And when the bells toll, you'll remember me
Bells toll when the clock hits a specific time.
"I always accompany the devourer of sons" - Cronus, the devourer of sons, is a titan that is associated with the personification of time. This could also explain the title, since Cronus's action made him seem like a monster.